Concentration of nitric acid



Dec; 13, 193s.A F. CARL 2,139,721

l K CONCENTRATION NITRIC ACID Filed March 23, 1935 Sheets-Sheet 1 vhamm/05 I LLM INVENTOR. Fred Carl ATTORNEY.

CONCENTRATION OF NITRIC ACID fred Caz- ATTORNEY.

Patented Dec. 13, 1938 UNITED STATES CONCENTRATION OF NITRIC ACID FredCarl, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company,Wilmington, Del., a corporation of Delaware Application March 23, 1935,Serial No. 12,563

11 Claims.

The present invention relates to an improved process of concentratingnitric acid, and more particularly to such a process in which a minimumamount of dehydrating agentis required.

6 This invention also relates to the denitration of spent nitrationacids and to the production of concentrated residual acid therefrom.

It has long been recognized that dilute nitric acid can not beconcentrated above 68% strength by distillation without the use of adehydrating agent. For this purpose, sulfuric acid is most commonlyemployed, but other materials such as anhydrous sodium nitrate, calciumnitrate, sodium polysulfate, phosphoric acid and arsenic acid have alsobeen suggested.

Of these dehydrating agents, sulfuric acid has been employed in twogeneral types of procedures, namely, (1) batch processes and (2)continuous processes. According to the batch procedure, a mixture ofsulfuric acid and dilute nitric acid is placed in a simple retort andthe nitric acid distilled. The rst fractions of the distillate areconcentrated acid, but, since successive fractions are progressivelyweaker, the batch procedure has been largely replaced by more efficientcontinuous methods.

The continuous process of Pauling (disclosed in U. S. Patent 1,031,865),for example, introduces a mixture of strong sulfuric acid and weaknitric acid continuously at the top of a packed tower. At the same time,a countercurrent of steam is introduced at the base of the tower, andthe interaction of the ascending steam and the descending acidsgenerates the heat required to volatilize the nitric acid. Subsequentprocesses along the same lines have covered methods of generating thesteam from materials employed within the system.

The most serious objection to any method employing steam is the dilutingeffect of the watervapor on the residual sulfuric acid. To minimize orcompletely avoid this effect, hot gases other than steam have beenproposed as counter-current heating media. Thus steam mixed with air,air followed by a final operation using steam, or air used alone, havebeen suggested. Nitrous gases have also been advanced for the samereason. While these media produce a stronger re- 50 sidual sulfuricacid, they have not been widely employed in the art because of thesomewhat greater efficiency of steam as a heating and denitratingmedium. All known processes of concentrating nitric acid, therefore, aredisadvantageous either in the poor thermal efficiency of thecounter-current medium or in the diluting effect of this medium on theresidual sulfuric acid.

The object of my invention is a new and improved continuous process forconcentrating nitric acid. A further object is a thermally efficientprocess which produces a stronger residual sulfurie acid. A stillfurther object is a process of concentrating dilute nitric acid withoutthe use of steam as a counter-current heating medium. An additionalobject is a process of denitrating spent nitration acids. 4Other objectswill become apparent as the invention is hereinafter described indetail.

AI have found that the foregoing objects are accomplished by the use ofa thermally efficient gaseous counter-current medium which does notdilute the sulfuric acid, but, on the contrary, actually increases itsconcentration. I have found that sulfur trioxide is satisfactory as thecounter-current medium. 20

'Ihe dilute nitric acid and the strong sulfuric acid are rst preheatedaccording to my invention and then added separately to the top of thedehydrating tower. The heat content of the separate acids, plus the heatliberated when the two acids mix, is sumcient to volatilize much of thenitric acid in the mixture. The remaining nitric acid is then driven offby introducing gaseous sulfur trioxide into the bottom of the tower. Inthis manner a large amount of the heat usually lost is employed, and theresulting sulfuric acid residue is stronger, since sulfuric acid isformed in the process. The advantages of this method will now beapparent, since the resulting residual sulfuric acid according to thisprocedure is of greater strength than that resulting from prior artmethods.

An equipment or apparatus suitable for carrying out the presentinvention comprises a standard packed nitric acid dehydrating tower orother equally satisfactory apparatus which will accomplish the desiredresults. Also, if preferred, an oleum still may be used as a suitablesource of sulfur trioxide in conjunction with a nitric acid dehydrator.In this case, the residual overow of the oleum still runs into the topof the packed dehydrating tower, while the vapor line conducts v gaseoussulfur trioxide to the bottom of the tower. The vapor line may beprovided, if desired, with a cooled section in which a portion of thesulfur trioxide is condensed and withdrawn as liquid sulfur trioxide.

In order .to disclose my invention more fully, reference is made to theaccompanying diagrammatic representations of preferred embodiments of myinvention. These are to be regarded solely as illustrative only, and arenot to be taken as limitations of the scope of the invention.

Referring to the drawings, Figure 1 represents a vertical elevation of anitric acid dehydrating tower as contemplated by my invention.

Figure 2 represents a similar View of a dehydrator in combination withan oleum still constructed in accordance with my invention.

The same letters are used in the drawings and the accompanyingdescription, to indicate corresponding parts of the respective views.

Referring particularly to Figure 1, A represents a standard type nitricacid dehydrating tower packed with an acid-resisting material B. Theupper end of the tower is provided with a pipe C leading to any suitablenitric acid condensing system; a pipe D for introducing the pre-heateddilute nitric acid; and a pipe E, through which the pre-heatedconcentrated sulfuric acid is introduced. The base of the tower isprovided with a pipe E for admitting the sulfur trioxide, and a trappedline G for discharging the residual sulfurie acid. The pipes may befitted with valves, as desired.

In operation, dilute nitric acid and 101% sulfuric acid are introducedinto the tower through the pipes D and E, respectively. These acids arepre-heated to a temperature of approximately 110 C. so that the mixingof the preheated weak nitric and the strong sulfuric acids generatessufficient additional heat to cause the temperature of the mixture torise above the boiling point of the nitric acid, most of which distills.The residual acid, containing a small amount of nitric acid, then flowsdownward over the acid-resisting material B and comes in contact with anascending counter-current of sulfur trioxide, admitted through the pipeF. The sulfur trioxide combines with the water in the mixture to formsulfuric acid, thereby generating suiiicient heat to cause the lasttraces of nitric acid to distill. The vapors of the nitric acid pass;out of the tower through the pipe C leading to suitable condensingcoils, where approximately nitric acid is condensed. The residualsulfuric acid passes out through the trapped line G, from whichapproximately 74% sulfuric acid is discharged into a storage tank. Theresidual acid thus obtained is generally more than 6% stronger than thatobtained when steam is employed as the countercurrent heating medium.

Referring now to Figure 2, the dehydrating tower of Figure 1 isconnected with an oleum still I-I. Fuming sulfuric acid is introducedinto the oleum still through the line J from the source of supply.Gaseous sulfur trioxide is distilled through the line K which is joinedto the opening F at the base of the dehydrating tower A. A portion ofthe line K may be trapped and cooled, if desired, and liquid sulfurtrioxide may be drawn ofi through the valve L. The residual sulfuricacid from the oleum still passes through the trapped line Minto theupper part of the dehydrating tower A through the pipe E. Dilute nitricacid is supplied to the dehydrator from a source of supply through theline D which is surrounded by a steam jacket N. The oleum still islikewise heated by the steam jacket O.

In operation, fuming sulfuric acid (104%) is continuously passed intothe steam jacketed single tube oleum still H in which it is heated t0about 195 C. At this temperature, a portion of the free sulfur trioxideis vaporized and passes through the S03 vapor line K to the bottom ofthe dehydrating tower A. 'I'he resulting hot residual sulfuric acid(101%) in the oleum still H overows into the top of the nitric aciddehydrating tower A, through the trapped line M. At the same time,dilute (60%) nitric acid, heated by the steam in the jacket N to aboutC., is added in the correct amount to give a resulting mixed acid ofabout 42% HzSOi and 35% I-INOa when mixed with the 101% H2SO4 from theoleum still. The heat content of these two acids, plus the heatgenerated by dilution, vaporizes the greater part of the nitric acidwhich passes out of the dehydrating tower through the line C to thecondensing system.

The mixed acids flow downward over the acidresisting packing B and comesin contact with the ascending current of S03 added at the bottom of thetower. The S03 combines vigorously with the water in the mixed acid andthe heat liberated completely Vaporizes the remaining traces of nitricacid. The vapors of nitric acid leaving the still through the line C arecondensed as strong nitric acid (about 95%). The denitrated residualsulfuric acid (74%) flows from the bottom of the tower through thetrapped line G. The acid thus produced is substantially stronger thanresidual acids obtained from previous methods.

If it is desired to produce oleum distillate, the charging acid for theoleum still is increased in strength and the excess S03 is condensed anddrained from the system through the valve L as liquid sulfur trioxide.

A similar procedure may be used for denitrating spent nitration acids,or mixed acids containing small amounts of nitric acid. In this case,the pre-heated spent acid is added in place of the 60% nitric acid inthe above examples. Not only may the spent acid be completely denitratedby this process, but the residual sulfuric acid is partly reconcentratedby the treatment. Moreover, the traces of nitric acid in the spent acidmay be recovered at the same time.

While I have described my invention with particular reference to theconcentration of nitric acid, it is apparent that the same procedure maybe employed to concentrate or dehydrate other volatile materials. Thus,for example, bromine in water may be concentrated in a similar manner.Likewise, other volatile acids such as acetic acid, hydrochloric acid,and the like may be obtained in an anhydrous condition by a similarprocedure. Moreover, it is not essential to use sulfuric acid as thesole dehydrating agent, for any liquid dehydrating agent, such asphosphoric acid, may be employed with sulfur trioxide as thecounter-current heating and denitrating medium. In general, any Volatilematerial may be dehydrated or concentrated in accordance with myinvention, provided it is inert with respect to sulfur trioxide andsulfuric acid or similar liquid dehydrating agent.

From the foregoing detailed description of my invention, it will beapparent to those skilled in the art that many variations may be made inthe process without departing from the spirit or scope of the invention.I therefore intend to be limited only as indicated in the followingpatent claims.

I claim:

1. The process of concentrating aqueous solutions of volatile acidsselected from the group consisting of nitric, hydrochloric, and aceticacids, which comprises establishing in a common path a current of asubstantially non-volatile uid dehydrating agent selected from the groupconsisting of sulfuric and phosphoric acids and a counter-current flowof gaseous sulfur trioxide, introducing into the common path a currentof said aqueous volatile acid in co-current flow with the dehydratingagent, utilizing simultaneously the dehydrative capacity of sulfurtrioxide for concentrating the dilute solution and the heat of hydrationof sulfur trioxide for distilling the vapors of said volatile acid in asubstantially dehydrated condition, and withdrawing the vapors of thevolatile acid in concentrated form.

2. The process of concentrating aqueous solutions of volatile acidsselected from the group consisting of nitric, hydrochloric, and aceticacids, which comprises establishing in a common path a current of apreheated substantially nonvolatile fluid dehydrating agent selectedfrom the group consisting of sulfuric and phosphoric acids and acountercurrent flow of gaseous sulfur trioXide, introducing into thecommon path a current of preheated aqueous volatile acid in cocurrentflow with said dehydrating agent, utilizing simultaneously thedehydrative capacity of sulfur trioxide for concentrating the dilutesolution and the heat of hydration of sulfur trioxide for distilling thevapors of said volatile acid in a substantially dehydrated condition,and withdrawing the vapors of said volatile acid in concentrated form.

3. The process of concentrating aqueous solutions of volatile acidsselected from the group consisting of nitric, hydrochloric and aceticacids, which comprises establishing in a common path a current o-fstrong sulfuric acid and a counterflowing current of gaseoussulfur`trioxide, introducing into the common path a current of saidaqueous volatile acid in co-current flow with the sulfuric acid,utilizing simultaneously the dehydrative capacity of sulfur trioxide forconcentrating the dilute solution and the heat of hydration of sulfurtrioxide for distilling the vapors of said volatile acid in asubstantially dehydrated condition, and withdrawing the vapors of thevolatile acid in concentrated form.

4. The process of concentrating aqueous solutions of volatile acidsselected from the group consisting of nitric, hydrochloric, and aceticacids, which comprises establishing in a common path a current ofpreheated strong sulfuric acid and a counter-ilowing current of gaseoussulfur trioxide, introducing into the common path a current of saidpreheated aqueous volatile acid in co-current ow with the sulfuric acid,utilizing simultaneously the dehydrative capacity of sulfur trioxide forconcentrating the dilute solution and the heat of hydration of sulfurtrioxide for distilling the vapors of said volatile acid in asubstantially dehydrated condition, and withdrawing the vapors of thevolatile acid in concentrated form.

5. The process according to claim 3, in which the sulfuric acid containsdissolved sulfur trioxide.

6. The process of concentrating dilute aqueous nitric acid whichcomprises establishing in a common path a current of sulfuric acid and acounterflowing current of gaseous sulfur trioxide, introducing into saidcommon path a current of7 dilute aqueous nitric acid in coi-current flowwith the sulfuric acid, utilizing simultaneously the dehydrativecapacity of sulfur trioXide for concentrating the dilute solution andthe heat of hydration of sulfur trioxide for distilling the vapors ofsaid nitric acid in a substantially dehydrated condition, andwithdrawing the vapors of the nitric acid in concentrated form.

7. The process of concentrating dilute aqueous nitric acid whichcomprises establishing in a common path a current of preheated sulfuricacid and a counter-owng current of gaseous sulfur trioxide, introducinginto said common path a current of preheated dilute aqueous nitric acidin co-current flow with the sulfuric acid, utilizing simultaneously thedehydrative capacity of sulfur trioxide for concentrating the dilutesolution. and the heat of hydration of sulfur trioxide for distillingthe vapors of said nitric acid in a substantially dehydrated condition,and withdrawing the vapors of the nitric acid in concentrated form.

8. The process according to claim 6, in which the sulfuric acid containsdissolved sulfur tmoxide.

9. The process of concentrating dilute nitric acid which comprisesheating fuming sulfuric acid to a temperature at which at least part ofthe sulfur trioxide distills, introducing the residual strong sulfuricacid resulting from this treatment, while still hot, into the upperportion of a dehydrating chamber, simultaneously introducing apre-heated dilute aqueous solution of nitric acid into the upper portionof said chamber, passing i the gaseous sulfur trioxide distilled fromsaid firming sulfuric acid into the lower portion of said chamber,withdrawing the vapors of concentrated nitric acid from the upperportion of the chamber, and withdrawing the' residual sulfuric acid fromthe lower portion of the chamber.

10. The process of denitrating sulfuric acid containing nitric aci-d,which comp-rises preheating sulfuric acid, establishing in a common pathwith said heated acid a counter-flowing current of gaseous sulfurtrioxide, introducing into said common path a current of the acid fordenitration, in co-current ow with the sulfuric acid, withdrawing vaporsof nitric acid in concentrated form, and withdrawing a residual sulfuricacid in dilute form.

l1. The process of denitrating spent nitration acids which comprisesheating fuming sulfuric acid to a temperature at which at least part ofthe sulfur trioxide distills, introducing the residual sulfuric acidfrom this treatment, while still hot, into the upper portion of adehydrating chamber, simultaneously introducing pre-heated spentnitration acid into the upper portion of said chamber, passing thegaseous sulfur trioxide distilled from said furning sulfuric acid intothe lower portion of the chamber, withdrawing the vapors of concentratednitric acid from the upper portion of the chamber, and withdrawingresidual sulfuric acid from the lower portion of the chamber.

FRED CARL.

